When I import some mp4 files to AE 2021 it only reads the first two seconds (one of the files duration is 1'17''), if the same file is imported to AE 2020 it works fine. The same thing happens when the files are imported to Media Encoder 2021 and Premiere 2021.
Without any specific info about the file specs such as how they were created, what their timecodes and streams are and so on nobody can tell you much. As a start actually check the footage interpretation. AE may simply have set a wrong start TC or something like that based on the files' metadata.
The files are in this case downloaded from Frame IO, Full HD (H.264/MPEG-4 AVC). AE 2021 has the same footage interpretation has the AE 2020 (the default for this files 25fps, with a start time code at 0'00''00) for example one file has a duration of 1'17'12f and it only shows 2'', another one has a duration oh 4'12'05f.
I just dont understand why on AE20 works fine, and on AE21 this happens.