Hi and welcome to the forum:
First and foremost, Canon - like every other camera maker - makes absolutely no warranty and takes no responsibility for the performance of 3rd party equipment attached to their cameras. It is up to those 3rd party suppliers to make their equipment compliant to the design parameters of the OEM.
This is not new, and when you think about it, it is perfectly logical:
An OEM is not going to provide their IP to others to make competitive products until they have released their own equipment range - they usually spend many millions of dollars to produce new designs and they rightly want to get back that investment. There are indications that both Canon and Nikon have been engaging with 3rd parties to set up licensing agreements for those makers to proceed with designs on a case by case basis, which in the context of design integrity alone, is logical.
When a whole new lens mount and camera design system comes out, while an OEM will have done as much as possible to nail down the compatibility of their camera bodies with their new lenses and, in Canon's case, with their legacy lenses, on-going tweaks will occur. There is evidence that this is a dynamic issue, with multiple firmware updates coming out since the R-series MILCs were released - both for bodies and lenses. Asking 3rd party suppliers to deal with this will simply complicate matters, and put those 3rd party suppliers at risk.
No OEM has the right or responsibility to determine the design of 3rd party equipment without a licensing agreement that dictates the rights and responsibilities of the two parties for the design of a product. Design is the right and responsibility of those suppliers. Thus 3rd party equipment is the sole responsibility of that manufacturer.
A lot of people have compared the state of Canon's 3rd party lenses to Sony, saying there are lots of 3rd party lenses out there. However, if one looks back to the same period since 2010 when Sony introduced their E-mount, they allowed very, very few 3rd party lenses for at least 6 years. I went to the effort of logging the release date every 3rd party lens marketed for Sony and comparing the two, Canon is actually ahead of Sony in that respect. The floodgates for Sony 3rd party lenses opened up from early 2017.
There are some 3rd party lenses out there, but the more sophisticated designs are very likely in the pipeline - remember, designing the optics and software for these much more sophisticated interfaces has its own design period that takes time. There are hints that Sigma may announce some of their lenses for the RF mount this month.
In the meantime, you can engage with the makers of 3rd party legacy equipment to see if they can offer firmware updates to make their gear more compliant to the new interface. As my respected colleague Kevin has said, all you can do right now is turn off the OIS and contact the lens makers.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
