Hello,
I am not entirely sure if this is a bug, but I thought it might help you in case it really is. If it is not, please could anyone explain to me what is happening there? I am attaching scene with simple example and information on how to reproduce certain issues.
example (6.8 KB)
(using Windows 10 & TD 2020.25380)
Thanks
In this case, in your Movie File In TOP, you have ‘Always Load Initial Frame’ turned off.
This means there may be some frames in which the new movie has not yet loaded.
It will report a length of 0 in that case.
I would leave this option as is (as it avoids frame drops), but modify your logic accordingly to check the movie load status. (example the ‘open’ info channel).
Cheers,
Rob.
Aha, I see. Thank you very much for explanation, that makes sense now. Sorry for posting in bugs - I thought there is some hidden bug causing this behavior. 
Not a problem! Wasn’t obvious.
Thanks for posting.