Two potential causes.
1. You've enabled stickykeys. This is completely unrelated to Blockland. It's a Windows 'feature'. Control Panel > Accessibility > Disable sticky keys, then, as further steps, go into each of the 'accessibility feature's settings, and disable all the shortcuts.
2. Shift is 'permapressed' in Blockland. This could happen from lag, or alt tabbing or clicking off screen while shift is held. Cancelled by just hitting shift again. This isn't a bug specific to Blockland either. When you find typing in anything isn't working quite right, just tap shift, ctrl, and alt. Usually resets them.