I am using Ubuntu 20.04 and the problem is present in both the appimage and the flatpak versions of Krita 4.4.2. I have tested with the 4.4.1 appimage and there is no issue there.
When loading an (existing) comicConfig.json (56 pages, 4962x7014 pixels) comic, Krita displays a “Loading pages” dialogue for ~10 seconds before it loads the thumbnails. When that is done, Krita uses 7.5GB more RAM than before opening the json file and it doesn’t seem to be released. Opening any of the pages uses additional memory still.
In 4.4.1 opening comicConfig.json is instanteneous and uses imperceptible amounts of additional RAM.
I believe it might be because Krita loads those files as KisDocuments (which means it loads it fully) and then it doesn’t release it even though it removes those documents because Krita doesn’t release memory unless it’s needed (technicalities of using TileManager).
I think in other places Krita just loaded mergedimage.png, I think it would be better if it used it everywhere for thumbnails unless it’s for actual loading the file when the user clicks on it.