Without sight of your Project / Model and sample data, nothing obvious comes to mind.
However, if I were debugging one of my own projects to solve this problem, I would consider the following steps:
Make sure that this Summary doesn't already have a filter applied. I think this would probably (i.e. should!) prevent you from adding a further filter at the Export stage.
Take a close look at the filter in question. Have any of the fields used in the filter been renamed, 're-typed' or deleted?
This could include a calculated field which has dependencies elsewhere in the model.
Upgrade to V12.4 - the current version includes a number of updates which may have addressed any issues related to this problem - or at least may provide a more meaningful indication of potential conflicts or dependencies. From my own experience, I would recommend this upgrade as a matter of course. I'm currently using V12.4 and I'm very happy with it.
I hope this helps. If it's possible to share the model and some sample data, for example via a PM, I would be happy to take a closer look.
Apparently, this behavior was a fluke because when I went back to look at it again, the Filter dropdown was no longer grayed out. I've also been able to apply a filter to a summary and export successfully (without using the advanced options). This didn't work either a few days ago.
Your suggestion to upgrade to 12.4 is well taken. Thank you!