#38597 is caused by #35864.

To reproduce this issue, at least it is required four different models
and three left joins from different relations.

When merging a relation from different model, new stashed (left) joins
should be placed before existing stashed joins, but #35864 had broken
that expectation if left joins are stashed multiple times.

This fixes that stashed left joins order as expected.

Fixes #38597.