Why could my opaque PETG be printing "transparent" in certain places?
Are you using Z-hop? Is there any play in the Z-axis direction? It appears that parts of the first layer are printed much thinner than other parts.

What can happen if there is a little play in the Z-axis direction that the nozzle doesn't return to the same level after a Z-hop movement (e.g. backlash in the leadscrew nuts).

The "transparent" printed part appears thinner, this must indicate that the Z positioning is not up to par.