REALLY long post I'm sorry

I have zero clue how this happened, but my trans doesn't appear to have any fluid! I checked it a month ago, and I could still see it on the stick. Maybe a tad on the low side, but not enough where I needed to fill it.

There is NO leak, checked everywhere, keep cardboard on the ground specifically to keep an eye out for leaks of any kind.

My transmission has not had any issues thus far. The reason I was checking at all was because I have a vibration when I flick my gear vendors on in 3rd over. (This is NOT related to speed, there's no vibration when overdrive is off, so I do not believe its converter shudder)

What long term issues could I have from this? If I caught this before I had major issues, am I likely safe?

Also, could this be the reason why power from my new engine is not reaching the ground? Its very sluggish but I got good spark, good looking sparkplugs and proper fuel pressure so I'm confused

ADDITIONAL CONTEXT:

It is a th400 with a manual valve body

No trans cooler, but upgraded radiator with twin electric fans, 3000 CFM.

I have not driven this car a lot. This weekend was the first time I've actually started to use it more. Estimated 30 miles on the new engine with this torque converter.

Last year, I installed the new engine, but it quickly failed due to an error on the builders part. The engine was removed, torque converter stayed in place. Didn't think there was much spillage, but i did NOT top off fluid after reinstallation. Dipstick read ok so I didn't top it off. However, I did not account for fluid drizzling from the radiator and lines

Not sure how it got this bad but need some questions answered