Who I have the same questioned this topic

I am having the same issue.   Messages are sent from an AT&T iPhone to my Verizon Moto Z Force Droid in Black/Lunar Grey 32 GB Black and my default messaging app is Message+.  Many are broken into 67 character messages, and in the wrong order, but it does not always break the messages up.

It seems to me like if there is emoji in the message it will break it up, into 67 byte messages (long press the message and tap Message Info to get the length).

But I also got a message that I did not see any emoji in it, that appears it should have been 233 characters but was broken up into one message of 153 characters and another of 80 characters.  And another that was a total of 486 bytes and was broken up into 3 messages each 153 and one that is 27.  I also have messages that were broken in the middle of a word into lengths of 153 and 46 bytes and came across in the wrong order.

I also have one that was 156 bytes and it was not broken up, but I got it twice.

And I have one that was 159 bytes that came across just fine.

I have also received multimedia messages of 263.4 KB that with a relatively long text message that appears to have come across just fine.