h.264 versus Quicktime export issue in Media Encoder

I am working in a project where I am creating compositions in the below specs.

2000x90

2000x70

I am finding that I can export the 2000x90 compositions from After Effects into Media Encoder with no issues, using the h.264 (Match Source - High Bitrate) format.

When I go to export the 2000x70 version, Media Encoder hangs and brings my computer to a grinding halt - slow, not responsive. This requires a force restart to fix.

I have worked out that if I export the 2000x70 in Quicktime (Prores 422), that it works, so it seems to me there is some sort of constraint on the h.264 codec in this composition size? It is a bit strange to me as I am using a brand new Mac Pro (specs below) and seem to have no issues exporting the 2000x70 comp in h.264 on iMacs and Macbook Pros I have had access to.

Can anyone explain this or find a workaround, other than exporting in Prores and then using a third party app to convert?

Computer Specs

Big Sur (11.5.1)

Mac Pro (2019)

96GB RAM

AMD Radeon Pro W5700X 16GB gfx card