There could be several reasons why your boyfriend's mom put the dishes back in the sink after you washed them:
Habit: It's possible that putting dishes in the sink after they've been washed is a habit she developed over time, and she didn't intend to undermine your effort or offend you.
Cleaning preferences: People have different cleaning preferences and standards. Your way of washing dishes might not align with how she prefers them to be washed or organized.
Drying or re-washing: She might have thought that the dishes were not dry enough or were not cleaned to her satisfaction and decided to rewash them or let them air dry.
Communication breakdown: Maybe she didn't communicate her expectations clearly, and you unknowingly did something that didn't align with what she had in mind.
Control or boundary issues: In some cases, individuals may display controlling behaviors or have boundary issues, leading them to redo tasks even if they were completed correctly by someone else.
Thankfulness or politeness: It's possible she wanted to show gratitude for your efforts by finishing the task herself, not realizing it might come across differently.
To address this situation, it's essential to communicate openly and respectfully with your boyfriend's mom. You can ask her if there was anything wrong with how you washed the dishes or if there's a specific way she prefers them to be handled. Clear communication can help resolve any misunderstandings and prevent similar situations in the future.