One things that causes blockage is a strange situation at the bottom of the Teflon tube in the hot end.
Difficult to explain but I will try...

Say you have a nice smooth bore all the way to the end of the nozzle. This means that the melted plastic has nowhere else to go to. The exit-end of the Teflon sleeve in the hot-end is perfectly flush with the threaded nozzle (and I am assuming there is one). You would think that soft plastic would fill up into the Teflon tube up to about where the cooling fins start. The melt would never be larger than 2mm in diameter. This the heater would easily maintain a melt regardless of flow rate, within reason. This is the ideal condition.

Now say we are having trouble with this particular factor. There are two things to check, but first let me try to explain the condition:
Say there was a gap between the top of the nozzle and the Teflon lining. This is the region that has a lot of melted plastic but this time, the plastic has filled the void between the nozzle and the Teflon liner. Say that the temperature toward the outer edges of the 4mm diameter void was not getting hot fast enough to remain fully soft/flowing. This becomes a cooling factor and begins to restrict/constrict the actual size of the remaining aperture. No longer 2mm in diameter, but rather a donut shape that may have a 1mm area that is insufficient to allow the expected flow to continue. The only term this is related to that I can come up with is sphincter. This condition can quickly lead to filament stripping causing thinning of the extruded PLA which is often seen in prints by appearing like skipping extrusion in 1-2mm long segments within the layer. Sometimes it resumes if the print slows its extrusion rate, and other times it just gets worse when a lot of filament is attempted to be pushed though. I get into detail here because this is a prominent problem in my printer type if things are not properly set up within this very narrow scope that we are talking about here. I don't know for a fact that this happens in your particular hot end. I do know that other printers do suffer from this scenario as it has been commented on before in various forums or videos. Basically, the junction between the inlet side of the hot-end and the Teflon liner is a design critical element. Again, this assumes you have a Teflon liner in your hot-end that can be maintained by the user.

The second scenario building on this previous thought; What if that Teflon liner is loose in the hot-end and has the ability to travel up and down within the hot-end say 2mm or more. This can happened when the print does retractions. The liner would stick a little to the filament being withdrawn and pushed back. You may have a similar problem to the filling of the gap but it is on the verge/edge of failure and basically healing itself and finally getting into a mode of catastrophic failure by no longer melting the oversized bulb of plastic. Next time you print, the preheat melts the bulb and things go back to pumping the Teflon sleeve back and forth. The difference here is that the preheating before a new print basically returns you to a known state, but the problem is lurking in the background. Basically, the Teflon liner should never have this issue that I know of. However, people with more experience than I might be able to help more in this regard.

Good to know that only PLA has been run through your machine. That makes the event of contaminant clogging less likely. Of course, a different know-good nozzle would work to do the same test. I am not going to say that what I describe here -is- the problem. I have experience with a lot of clog scenarios including a few little known failure modes that can only be related to specific filaments. Not having a temperature dial on my printers give me a solid baseline in all experiments. And until recently, my nozzle configurations are very different except the fact that they use the Teflon liner just above the nozzle. What I cannot say is how common a clogged nozzle condition is for your type of nozzle.

Obviously there has to be an answer. I can only give hints to where else you might look. As long as you are willing, I can certainly continue to scratch my head along with you. Most often, someone will chime in with a real nugget of wisdom directly related to your printer and problem. And I take my hat off to them for bringing it forward. Good luck, Machumara.