[nokinnikon]

Consider, some unlikely some likely:

Defective pressure plate, Dual mass fly wheel, clutch plate out of the box.

Bad cable/master/slave(concentric to throw out bearing improper bled/cable (clutch pedal stuck on floor)

Contaminated spots on clutch plate (oil, grease, residual penetritive sprays/oils on hands)

Flywheel has hot spots

Worn, defective thrust bearing/concentric or other.

Improper torquing of pressure plate/DMF.

Needs re-bleeding.

Try starting off in 4th gear a few times to maybe even out or to scuff off detritus on components, a long shot but you never know.

Engine/gbox mounts nice and aligned/torqued?

If it was me I would forensically take it apart again and step by step check/look for any improper fitting loose components, hey you might get as far as a gearbox mount and its loose and needs retorquing and that's it. Long shot but hey...

Some may say improper alignment of clutch plate, if that was so the splined/grooved input spigot/shaft from gbox to engine would not fit past the clutch plate. I've never had that problem I've always used an appropriately sized deep socket on an extension bar to align clutch plate, never had a failure. I hope this helps in any way shape or form.

Oh for the record if you or whoever does the clutch/gbox out again and it's all reassembled and you find the car often jumps out of gear 1st/3rd usual culprits it's almost always a badly aligned hose that can spit the car out of gear, pending on what pressure the hose is at at that time. Hose runs across the top of gbox and above the selector arm.