Would you mind loading v1.09 FW and testing the same process please?
Upgraded the firmware to the new beta version: DIR-890L HW:A1 FW:1.09.
After the firmware upgrade I did a factory default reset to the router. I completed the setup initialization of the router. Clicked on Firewall -> IPv4 Rules. The Remaining rules on the bottom of the screen changed from 15 to 24.
1) I create the rule listed above and it changes from Remaining 24 to 23 and lists the rule I created in the table.
2) I click the Save button and the rule I created is deleted and Remaining goes from 23 back to 24.
The upgraded beta firmware doesn't appear to correct the issue.