[Unclenick]

I had to reread to the OP. I misunderstood part of your symptom. I'm going to guess that the original owner probably put a healthy rollover (relief) angle on the sear. When you press the trigger with the safety on, the back edge of the sear nose clears the hammer notch which then rests on that relief angle. When you release the safety, the hook pushes the sear forward against that angle, and the forward bounce is enough to prevent it returning fully under the half-cock notch, which then strikes the relief angle, as well, pushing the sear forward instead of capturing it. If there is no rollover angle this could still happen with a sharp sear nose (undesirable from the standpoint of a tendency to bounce out of the hammer hook) on the rounded edge of an improperly shaped hammer hook.

I would be very leery of the quality of the amateur trigger job, assuming that's what it is. Second, the fit of the thumb safety is obviously incorrect. This does not mean the frame is necessarily bad. It should fit a standard safety better, but is is also the case that if someone removes enough material from a sear nose (as in retrying and retrying to get it right), it gradually tips back further to engage, and that moves the feet forward of the thumb safety. A replacement hammer with a pin hole or a strut hole or hook not quite correctly located can do the same thing. Because all these interactive fit elements are involved, there are a number of replacements for the thumb safeties that are oversize and have to be filed to just be able to engage. Look for any aftermarket thumb safety that says it will require some fitting.