I just tested my Sigma 50A on my R5, three different apertures. It was working.

Something seems off.

But, you have also found another solution.

On transitioning over to RF....I am still primarily EF and do enjoy the drop-in filter adapter. Last year I started the transition unexpectedly with 2 lenses and I wanted, the RF 200-800 and RF 24-105 f/2.8. Both great and fit their purpose for exceptionally well (the 200-800 is probably my most used lens at the moment).

My transition took another step as I took advantage of a refurbished RF 50 f/1.2 being on a deep (~25%) sale. That arrived yesterday and, admittedly, I will pick up a 100-500 as soon as I see it at a good price point.