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Hey there, thanks for the question.
The timing belt replacement itself would not generally effect how the transmission shifts. That does not rule out the possibility that there was not a connector left unplugged in the process that could cause your symptoms.
Do you have any lights on the instrument panel while driving? or does the car operate normally with no lights, just the hard shift?
Sorry got pulled away but I am back.
the connector on the seatbelt
There is one plug for the buckle (this is what tells when the passenger has their seatbelt on)
there is another on the retractor which is the pre-tensioner.
do you want me to continue chat or text instead?
heres a picture of the plug in reference.