Илья, доброго времени суток!
Проблема не нова. На тутошнем форуме встречалась несколько раз.
https://www.opennet.ru/openforum/vsluhforumID6/16762.html
Сам недавно столкнулся, но пока не победил.
С ACL получилось?
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CSCsi30964 Bug Details:
Static NAT statement disappears from running-configuration  
Symptoms:
On a Cisco router performing NAT, static NAT statements may disappear from the
running-configuration during operation. Any new flows requiring translation via
the missing statement may fail.
Conditions:
- This problem was first experienced in IOS 12.4(9)T.
- It has only been reported for extendable, inside source static NAT statements
for TCP ports 80 (HTTP) and 25 (SMTP), with and without a route-map:

Examples:
ip nat inside source static tcp x.x.x.x 25 y.y.y.y 25 extendable
ip nat inside source static tcp x.x.x.x 25 y.y.y.y 25 extendable route-map nonat
ip nat inside source static tcp x.x.x.x 80 y.y.y.y 80 route-map nonat extendable

- After the statement disappears from the running-configuration, it is still
visible in the startup-configuration.

- Existing translations created before the disappearance are cached in the NAT
translation table and continue to work correctly.
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>[оверквотинг удален]
>
>CSCtf36262 , CSCtf36243
>Symptom:
>If an Access Control List (ACL) for the NAT inside local address
>and the NAT pool for the inside
>global addresses overlap, after a dynamic translation expires, the ARP entry for
>the inside global
>address is deleted. This results in the failure of outside-to-inside static translations
>that rely
>on a cached ARP entry.