By updating to in5 > v3.7.5, I noticed...
InDesign2021 in5 > ImageQuality > "Use ObjectExportSettings" > "JPEG 150DPI or 300DPI" do not apply to exported in5 images (all images are LowRes unless they are set to be 300DPI)
Interesting, PNGs are rendered OK.
As I skipped some updates, I can ensure it works fine in v3.6.9.
Please address this bug ASAP