Ok so I decided to do some testing. I made 2 testcases, one with raftbuilding tech and one without raftbuilding tech.
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CORRECT: Before raftbuilding, just improving the hemp (tile C, revealed and enabled) did not connect it. After raftbuilding, it was connected.
CORRECT: Before raftbuilding, improving the bison (tile A) AND building a trail there (but not in tile B) did not connect it.
ODD: Before raftbuilding, just improving the bison (tile A) did not connect it. However, building a trail in tile B did connect it. This is odd.
CORRECT: After raftbuilding, improving the hemp and the bison immediately connected them.
I will attach my 2 save games I used for testing, one with raft building tech and one without. It has gatherers standing by in all the relevant spots, and all trails and improvements can be made in 1 turn.