From unsigned to psbt transaction
The "suffix" is not a fixed suffix. The number of 00 bytes needs to match the number of inputs and outputs of the transaction. Those bytes are the terminators for the key-value maps for each input and output. The automatic converter does not fill in those maps, but an empty map will still have its null terminator. So the number of empty maps (and thus the number of null terminators) will change depending on the number of inputs and outputs in your transaction.

Also, those bytes will change as the PSBT gets filled with information. It is not a fixed suffix. So as the extra data for signing for each input and output gets filled, the "suffix" of your PSBT will change.