That's because you're not actually running Bash. /bin/sh points to the Toybox shell by default in Termux, despite Bash is the default user shell there. The similar applies, for example, to Debian, where /bin/sh points to Dash by default, to Mac OS X 10.0–10.2, where it pointed to tcsh, or to OpenWRT and bare Android, where it points to Busybox.
Generally, it's a bad practice to use the `#!/bin/sh` shebang line when you explicitly use Bash-isms (which $RANDOM belongs to) rather than targeting any POSIX-compliant shell.
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That's because you're not actually running Bash. /bin/sh points to the Toybox shell by default in Termux, despite Bash is the default user shell there. The similar applies, for example, to Debian, where /bin/sh points to Dash by default, to Mac OS X 10.0–10.2, where it pointed to tcsh, or to OpenWRT and bare Android, where it points to Busybox.
Generally, it's a bad practice to use the `#!/bin/sh` shebang line when you explicitly use Bash-isms (which $RANDOM belongs to) rather than targeting any POSIX-compliant shell.
4*(75*3-10+2) :P