PT-BR to spanish on V4 Flash

Hey everyone, quick question about V4 flash 0731 on OpenRouter. I usually chat in Brazilian Portuguese, but out of nowhere the model keeps switching to Spanish. I already set a strict system prompt telling it to never change the language, but it still does it anyway. I also tried lowering the temperature down to 0.4 and it made zero difference. At first I thought it was a client issue, but I tested it across Hermes Agent, LobeHub, LibreChat and Odysseus AI, and the exact same bug happens everywhere. When I switch to V4.1 flash the issue goes away completely, but the pricing is almost 10 times higher for me right now, so sticking with 4.1 is not really ideal. Has anyone run into this problem and managed to fix it somehow?