This is the output from vppctl show verbose:

  13 list pool elements
  192.168.20.10: 0 dynamic translations, 1 static translations
    i2o 192.168.20.10 proto tcp port 22 fib 0
    o2i 192.168.37.205 proto tcp port 22 fib 0
       external host 192.168.37.205
       last heard 2123.32
       total pkts 86246, total bytes 5523775
       static translation

  192.168.20.5: 0 dynamic translations, 1 static translations
    i2o 192.168.20.5 proto tcp port 22 fib 0
    o2i 192.168.37.207 proto tcp port 22 fib 0
       external host 192.168.37.207
       last heard 68260.35
       total pkts 74, total bytes 7737
       static translation

  192.168.20.7: 0 dynamic translations, 1 static translations
    i2o 192.168.20.7 proto tcp port 22 fib 0
    o2i 192.168.37.208 proto tcp port 22 fib 0
       external host 192.168.37.208
       last heard 2495.53
       total pkts 30, total bytes 4869
       static translation

  192.168.20.13: 0 dynamic translations, 1 static translations
    i2o 192.168.20.13 proto tcp port 22 fib 0
    o2i 192.168.37.203 proto tcp port 22 fib 0
       external host 192.168.37.203
       last heard 6425.74
       total pkts 20, total bytes 4353
       static translation

  192.168.20.12: 0 dynamic translations, 1 static translations
    i2o 192.168.20.12 proto tcp port 22 fib 0
    o2i 192.168.37.210 proto tcp port 22 fib 0
       external host 192.168.37.210
       last heard 6639.42
       total pkts 19, total bytes 4301
       static translation

  192.168.20.14: 0 dynamic translations, 2 static translations
    i2o 192.168.20.14 proto icmp port 54467 fib 0
    o2i 192.168.37.207 proto icmp port 54467 fib 0
       external host 192.168.37.207
       last heard 8110.31
       total pkts 6, total bytes 504
       static translation

    i2o 192.168.20.14 proto icmp port 58784 fib 0
    o2i 192.168.37.207 proto icmp port 58784 fib 0
       external host 192.168.37.207
       last heard 68456.46
       total pkts 6, total bytes 504
       static translation

static mappings:
local 192.168.20.14 external 192.168.37.207 vrf 0

There are multiple entries for 192.168.37.207, which results in incorrect translations:

18:57:39:345467: ip4-input
  TCP: 192.168.37.18 -> 192.168.37.207
    tos 0x00, ttl 64, length 60, checksum 0xd57d
    fragment id 0x990c, flags DONT_FRAGMENT
18:57:39:345474: nat44-out2in
  NAT44_OUT2IN: sw_if_index 2, next index 1, session index 1
18:57:39:345478: ip4-lookup
  fib 0 dpo-idx 5 flow hash: 0x00000000
  TCP: 192.168.37.18 -> 192.168.20.5
    tos 0x00, ttl 64, length 60, checksum 0xe747
    fragment id 0x990c, flags DONT_FRAGMENT

The configured static mapping is 192.168.20.14/192.168.37.207, but the actual translation is from prior session - to 192.168.20.5.