I already tried clearing all media cache, unfortunately that didn't fix it.
Exporting this project also leaves the transitions invisible, so its not just the project preview.
At the end of this video I reapply the transition animation to show an example of what they're supposed to look like, that had been applied to every transition between images. After I saved and closed the project, this is what I had upon reopening the project again.
I have previous, finished video projects that also used this transition effect, and I am able to reopen those projects with the transitions still in tact so this issue seems to be linked to this project save specifically.
Upon experimenting, it appears the issue is related to the masking effect? If I create a second mask on top of the first one, the layer is visible again. It seems this is a bug relating to corrupted masking properties upon saving and reopening the project...But all masking properties are getting corrupted every time I save, close, and reopen this project again.