Any concerns with APFS formatted Mac backuped to HFS formatted external hard drives?

So it is important to ensure that my Time Machine drive must remain formatted in HFS, am I correct?

Pretty much yes. Although I think it's difficult to accidentally upgrade the TM drive to APFS, short of erasing it first.

Also see https://support.apple.com/en-gb/guide/mac-help/mh15139/mac from which I got "Important: You can back up from an HFS+ or APFS-formatted disk to an HFS+ disk; however, Time Machine can’t back up to an APFS-formatted disk. If you select a back up disk formatted as APFS, Time Machine offers to reformat it as HFS+."

I'm not 100% sure but I think your main issue is to ensure that the Time Machine backup drive itself does not get converted to APFS. If this were to happen and for any reason, you had to revert your primary drive to Extended Journalled, Time Machine running on APFS won't work.

You are backing up your data and the metadata associated with your data (file dates, file ownership, file access permissions, file extended attributes, etc...), not the APFS file system's internal structure.

Just about every modern operation system can preserve a files's data bit-for-bit.

APFS and HFS+ both know about macOS specific file metadata attributes and can preserve them.

That is all a backup has to do. Exactly how APFS decides how to internally organize the files and metadata is not important. Just your data and macOS specific file attributes.