Oh it is actually a intended design. The game decrease the number by the view of the bombs.

Consider the following situations:
▫️💣
💣2️⃣0️⃣
For the 💣 on the top, it only see one digit,
💣
2️⃣
so it decrease that digit by one
💣
1️⃣
And then we handle the bomb on the left.
💣1️⃣0️⃣
 become
💣0️⃣9️⃣
The final result will be
▫️💣
💣0️⃣9️⃣

I think the confusion is caused by the orders of game evaluation bombs.
To avoid frustrating players, the game always evaluate bombs in orders that advantageous to the player.

Consider the following situations:
▫️▫️💣
💣1️⃣0️⃣

If we handle the top bomb first. It only see zero, it will be game over.
So we start from left bomb instead.

For the left bomb
💣1️⃣0️⃣
become
💣0️⃣9️⃣
Then for top bomb
💣
9️⃣
become
💣
8️⃣
The final result:
▫️▫️💣
💣0️⃣8️⃣

I think I should come up with more levels to explain this concept after game jam end.