remiere Pro 26.1 Beta – Ultra Key / Transparency broken in Program Monitor after update

Since updating to Premiere Pro version 26.1 (Beta), transparency appears to be broken in the Program Monitor when using Ultra Key, as well as other effects that rely on transparency.

Keyed areas display incorrectly or as solid instead of transparent, even though the effect itself is applied correctly.

Things already tested (no change):

• Sequence colour space changes, including Rec.709

• Nesting and un-nesting clips

• Switching renderer between GPU and Software

• Program Monitor settings

• Fresh project

• Restarting Premiere and clearing cache

This was working correctly before the update, so this appears to be a 26.1 Beta regression, not user error.

System:

• Windows 10

• NVIDIA RTX 4060 Ti

• Premiere Pro 26.1.0.010 (Beta)

Is anyone else experiencing this, or is there a known workaround?