Here is the "Errors and Faults" output from the two machines:

2019 Intel MBP (connects):

error   16:47:01.683167+0000    NEIKEv2Provider Bootstrapping; external subsystem UIKit_PKSubsystem refused setuperror  16:47:01.692737+0000    NEIKEv2Provider open flag(s) 0x01000000 are reserved for VFS use and do not affect behaviour when passed to sqlite3_open_v2error    16:47:01.692805+0000    NEIKEv2Provider cannot open file at line 46922 of [554764a6e7]error 16:47:01.692830+0000    NEIKEv2Provider os_unix.c:46922: (2) open(/private/var/db/DetachedSignatures) - No such file or directoryerror  16:47:01.870724+0000    NEIKEv2Provider [IKE_SA_INIT R resp0 7A1ACBA4CE3B8343-943C619482D68F79] Initiator init received notify error Error Domain=NEIKEv2ProtocolErrorDomain Code=17 "InvalidKEPayload" UserInfo={NSDebugDescription=InvalidKEPayload}error 16:47:02.483478+0000    NEIKEv2Provider Missing IPv4 subnet mask

2021 Apple Silicon MBP (doesn't connect):

error   16:48:23.692810+0000    NEIKEv2Provider Bootstrapping; external subsystem UIKit_PKSubsystem refused setuperror  16:48:23.709860+0000    NEIKEv2Provider open flag(s) 0x01000000 are reserved for VFS use and do not affect behaviour when passed to sqlite3_open_v2error    16:48:23.709939+0000    NEIKEv2Provider cannot open file at line 46922 of [554764a6e7]error 16:48:23.709959+0000    NEIKEv2Provider os_unix.c:46922: (2) open(/private/var/db/DetachedSignatures) - No such file or directoryerror  16:48:23.862999+0000    NEIKEv2Provider [IKE_SA_INIT R resp0 A13C3AD14E0D757D-38B3699D019D13AF] Initiator init received notify error Error Domain=NEIKEv2ProtocolErrorDomain Code=14 "NoProposalChosen" UserInfo={NSDebugDescription=NoProposalChosen}error 16:48:23.863059+0000    NEIKEv2Provider IKEv2Session[1, A13C3AD14E0D757D-0000000000000000] Failed to process IKE SA Init packet (connect)error  16:48:23.864311+0000    NEIKEv2Provider Failed to find suitable address, path supports IPv4 yes IPv6 no