It is a neutral problem...Something I have done is to eliminate the Customers equipment, by taking a piece of #4 thhn & split bolt it onto the POCO's service neutral, & connect it to the panel's neutral bar.
If the inbalance is still there, it will be a problem with the POCO's neutral connections.
If the inbalance straightens itself out, the hunt begins !!! but You will know that it is inside the Customers equipment.
The last time I did this trick, I found the problem went away, & voltage was good.
Called the POCO & told them THEY had a open neutral.....found out that a Squirrel had chewed the neutral down to the steel core out on the pole.