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Good day!

The one thing that draw my attention is the APIPA address used for the Sync interface. An intuition told me that most probably you cannot use APIPA as a static address for Sync. 

Quick check in a Lab shows that indeed we have ACTIVE/DOWN only because of the IP-addresses.

I have changed eth4-1(used for Sync) IP-address from

172.16.18.1/24 (23800_1) and 172.16.16.2/24 (23800_2)

to

169.254.1.50/24 (23800_1) and 169.254.1.51 (23800_2)

As a result, I got Active/Down from both ends after cpstop/cpstart. Your problem is replicated successfully. There is no MPDS used at all.

RFC 3927 states that 169.254.0.0/16 network is for automatic IP-address configuration. I may guess that Checkpoint follows the guideline and doesn't allow to configure an IP-address from this range manually.
Similar point is stated in sk179028

These IP subnets are reserved (you cannot use them in the CIN IP ranges):

0.0.0.0 / 8
127.0.0.0 / 8
169.254.0.0 / 16
192.0.2.0 / 24
224.0.0.0 / 4
203.0.113.0 / 24

Please, send my best regards to TAC engineers and don't make any unnecessary actions until you try to assign non-APIPA address for the Sync!