See also the official documentation:
Help Center > Owners Manual > Additional information > Topic threading algorithm

Topics and the Threading Algorithm

Posting by email with the same subject text causes them to thread together. Note, the comparison ignores prefixes like Re: that often are automatically added to replies and also ignores the group's subject tag.

If you post from your group's home page, just use New Topic (as opposed to Reply) if you wish to create a new thread.

Here is the threading algorithm:

If a message has threading information (i.e.: "References" and "In-Reply-To" entries in the message header), and the subject line in the referenced message is a match, use that.

If a message does not have threading information:

• If it's a New Topic web post (i.e.: not a reply), assume it's the start of a new thread
• If the subject starts with Re:, look for a matching subject within the last 30 days
• If the subject does not start with Re:, look for a matching subject within the last 2 days

Ref: https://beta.groups.io/g/main/message/12491

Note: for the above, the matching does not include standard prefixes like Re: nor does it include hashtags added to or removed from the subject. Please see https://groups.io/g/GroupManagersForum/message/18863 for information on how this interacts with topic locking.

Workaround for mail:

If posting by web is too inconvenient, your other option would be to vary the subject text slightly. Perhaps add the day of the week to the subject text, or something else to prevent each from being identical to another posted within the algorithm's date range.

Reminder -- if you are a Moderator, you can also split off messages. See the separate wiki page on that.