This problem is present at least since 5.0 probably before that too. Increasing spatial samples will cause some excessive memory usage and that memory isn’t freed even after done rendering.
You can sort of workaround the issue by using a combination of low spatial samples and more temporal samples in AA settings of MRQ.
However if your scene has difficult lighting setup then you might be out of luck as using low amount of spatial samples might not provide enough exposure in some dark areas.
Legacy sequencer output does not have this issue and if your scene/ animation does not invalidate path tracer then you should be able to render out stills with high amount of samples. On top of that the output is also much sharper than the one from MRQ (mrq applies some weird image pixel filtering or something where you just won’t get sharp image from path tracing)
I honestly don’t understand how is it possible for such issue to persist through multiple stable releases of the engine. I guess not many people care about path tracing for epic to look into this…