Hi Aram,
This is a side-effect of a known bug (UE-239584) that has already been fixed in UE 5.6. If you are building the engine from source, you can integrate the fix from CL 39761382 in ue5-main branch, or from SHA-1 ec9d0bef8887373ec0f0cd7ec37e40bc00a7b933 if you are using GitHub.
If you are not building from source and are unable to upgrade to UE 5.6, you can try to change the “FillStyle” property of the Progress Bar to “Scale” instead of “Mask”. This should behave better if the Progress Bar is not clipped by the viewport, but it will deform a fill image if you are using one.
Please let me know if this helps.
Best regards,
Vitor