I have come across similar contactor related problems but not in this specific truck.

However if the problem is happening on the same contactor tips the ideal thing to do would be to swap the motor cables around off the back of the contactor and then see if the fault then swaps over to the other contactor or remains on the same contactor.

If it remains on the same contactor irrespective whether it goes forward or backwards then there is a good chance its a contactor problem. On the other hand if the problem then transfers across to the other contactor its most likely going to be a motor related problem.

Its just unusual to have the problem occur in forward only if its a coating issue on the tips. You would have expected other tips to be affected.