Just steppin in here for the first time. It definitely sounds like you have a weird one. My first assumption is that something unusual has been inadvertantly turned on/off during your upgrades.
First, does this happen with all sets or just 1. It might be some specific option within the setup of the phone.
If just the one, I would delete and re-create to wipe the stored profile.
If more than one, I would start by checking recent additions to COS and compare them with default.
Below is a list of recently added COS options in dating back to about Rel 7.0 or thereabouts.
161 ACD Make Busy on Login
162 Call Park-Allowed To Park
163 Call Park Timer
164 Two B-Channel Transfer Allowed
165 Calling Party Name Substitution
166 Disable Conference Join Tone
167 Disable Executive Busy Override Tone
168 Return Disconnect Tone When Far End Party Clears
169 Suppress Redial Display
170 Call Forwarding Reminder Ring (CFFM and CFIAH only)
171 Suppress Delivery of Caller ID Display between Sets
172 Suppress Delivery of Caller ID Display between Sets - Override
173 Recall If Transferred to Original Call Destination
174 Allow Directed Call Pickup Of Attendant Call
175 Suppress Display Of Account Code Numbers
176 Maintain Ringing Party During Recall
177 Display Transfer Destination on Recall
178 Originator's Display Update In Call Forwarding/Rerouting
179 Called Party Features Override
180 Message Waiting - Disable Ringing Lamp Notification
181 Music on Hold on Transfer
182 Phonebook Lookup - Default to User Location
183 Phonebook Lookup - Display User Location
184 Phone Lock
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Occam's Razor - All things being equal, the simplest solution is the right one.