A bad ground is a frequent problem on those or burned out bulb is a possibility. Some replacement bulbs are made with pins just different enough they can fit in the socket but index improperly with the contacts so one other thing to look at.
If those are good, then the lamp is fed directly from the switch as shown here in the extracted diagram
https://www.packardinfo.com/xoops/html/downloads/wirediagrams/Turn%20Signal%20Extraction.pdf It is possible the new switch is warped and has a bad connection with the contact slide or the pin at the plug is not making good contact.
Some wheels have a pin on bottom of steering wheel which has to catch a little spring loaded hook or cam on the switch to cancel. If the wheel was reinstalled too high or shroud has slipped down so they don't connect, that can cause a cancel problem on those type switches. I don't remember if the 51 has that pin or not.
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