Now that I have been experiencing this error for a few days, I have checked your diagnosis with folders from other Microsoft accounts that were shared with me a long time ago, and have come to the same conclusion, which can be summarised as follows:
If the sharing OneDrive account and the OneDrive account adding the shared folder are both either equally 'old' or equally 'modern' platforms, the added folders will immediately appear in the receiving OneDrive account as folders embedded in the Windows Explorer application. However, if the platforms of the sharing and adding OneDrive accounts are different, i.e. one is 'old' and the other is 'modern', the added folders will appear correctly in the respective apps on iOS and Android and in the OneDrive web version, but not in Explorer on Windows (and I suspect not on MacOS either), where they will instead appear solely as a URL that can only be opened in a browser window.
However, as the folders shared by other OneDrive accounts and then added to My Documents are now displayed correctly in the web view, and this is also displayed correctly in the apps on iOS and Android, the error is probably not in the communication between the 'old' and 'modern' platforms themselves. Rather, it seems that the OneDrive app for Windows (and possibly the OneDrive app for MacOS) detects the platform type of the particular OneDrive account it is logged into and bases its further processing on it ('old' or 'modern') and is not compatible with the sharing syntax of folders of the other platform type. The lack of this ability to always work with both platform types, regardless of one's own platform type, is probably the reason for the malfunction.
I would like to go into more detail. I ran the above diagnostic on accounts that successfully shared folders with each other immediately after creation on all operating systems, including Explorer on Windows 10 Pro and Windows 11 Pro. The accounts I checked were also all created over ten years ago. In the meantime, the web sharing displays have been redesigned many times without any problems. So I'm guessing that a few years ago, or maybe just a few months ago, there must have been a different kind of change that, as your previous post answered here, did not just change the way the sharing functionality looked, but actually started to migrate some OneDrive accounts (or even the entire respective Microsoft accounts) to a structurally different platform. And since this all works for the OneDrive display on the web and in the respective OneDrive apps on iOS and Android, Microsoft seems to be aware of the problem and has implemented a solution that works. However, for the Windows OneDrive app (and possibly the MacOS one), no one at Microsoft seems to have bothered for months to achieve dual compatibility (i.e. simultaneous compatibility with both 'old' and 'modern' OneDrive platform types).
This gradual rather than simultaneous migration of OneDrive accounts seems to have been recognised as a problem with the sharing functionality and addressed on some platforms (web, iOS and Android apps). On the most productive platforms of all, Windows (and possibly MacOS), no attention is being paid to the relevant agents (e.g. the OneDrive app for Windows), and attempts are being made to turn a deaf ear and a blind eye to the problem, presumably hoping to wait until all accounts have been structurally migrated months or years later. For the lucky Microsoft365 subscribers, the whole thing has been garnished with a hefty price increase.