72 bug jumps out of 1st gear

Looked at a 72 bug today it wasnt registered so I didn't get a chance to take it for a drive. The owner said that it jumps out of first gear unless you hold it in.

They said their mechanic had a look and said it has had previous work done on it and mentioned the gearbox or something in the gearbox was replaced and possibly was put in wrong.

Does anyone know what could be causing it to jump out of first gear, and suggested cost of repairs best and wost case scenarios. 😂

Iv had two gear boxes do this in forth. On to my third gear box now. Can't get any clear answers myself. Have checked all external forces and ever changed internal detent springs( that didn't work)
First gear might be bent gear shift shaft, I'd check that before going inside the gear box.