PO distrib line budget status becomes Not Checked and encumbrance balance goes missing

We are on FSCM 9.2 Image 53.

Recently, several POs have had issues with their budget status which is mysteriously changing from Valid to Not Checked and the encumbrance balance is gone.

We are alerted to the issue when a voucher created from the PO receipt gets a budget exception and the exception is "Predecessor Not Found - RefDoc." When I look at the PO, I see the budget header status is Valid but the encumbrance balance is blank. When I go into the distribution lines, the budget status is Not Checked and the encumbrance balance is zero.

When I return to the PO header, the budget status has changed to Not Checked and the budget-check button is visible. I'm able to click the budget-check button to re-establish the encumbrance balance, which fixes the problem, but I don't know why the budget status changed. The PO could not have dispatched without a valid budget status, so something is happening to change the budget status on the PO. The most recent PO this happened to was not modified and hasn't been changed by anyone since it dispatched.