Can't create subscript inside subscript or superscript

[AllanChain]

Description

Can't use _ (Shift+-) or Ctrl+- to create subscript inside subscript or superscript.

Steps to Reproduce

1. Go to demo page https://cortexjs.io/mathlive/demo/
2. In the math input box, type a _ b _
3. Another example, type a ^ b _

mathlive-subscript-bug.mp4

Actual Behavior

2. The subscript b got selected
3. A subscript of a is created

Expected Behavior

Both: a subscript of b is created

Additional context

1. I can create superscript inside subscript or superscript
2. It's actually possible to create subscript inside superscript, but an extra subscript of a should be created before I can create subscript of b (see screen recording)

Environment

MathLive version 0.95.5

Operating System Linux

Browser Firefox and Chrome