different behavior of stdlib ostruct vs. ostruct-gem

I am on Ruby 3.0

With stdlib ostruct I get:

irb(main):002:0> x = OpenStruct.new
=> #<OpenStruct>
irb(main):002:0> x.varname1 = 'varval 1'
=> "varval 1"
irb(main):003:0> x.to_h
=> {:varname1=>"varval 1"}
irb(main):004:0> x.varname1 = 'varval 2'
=> "varval 2"
irb(main):005:0> x.to_h
=> {:varname1=>"varval 2"}
irb(main):006:0> x.varname2 = 'varval 3'
=> "varval 3"
irb(main):007:0> x.to_h
=> {:varname1=>"varval 2", :varname2=>"varval 3"}
irb(main):008:0> x.varname2 = 'varval 4'
=> "varval 4"
irb(main):009:0> x.to_h
=> {:varname1=>"varval 2", :varname2=>"varval 4"}

Starting with version 0.5.0 of the ostruct-gem: I get:

irb(main):001:0> x = OpenStruct.new
=> #<OpenStruct>
irb(main):002:0> x.varname1 = 'varval 1'
=> "varval 1"
irb(main):003:0> x.to_h
=> {:varname1=>"varval 1"}
irb(main):004:0> x.varname1 = 'varval 2'
=> "varval 2"
irb(main):005:0> x.to_h
=> {:varname1=>"varval 1", false=>"varval 2"}
irb(main):006:0> x.varname2 = 'varval 3'
=> "varval 3"
irb(main):007:0> x.to_h
=> {:varname1=>"varval 1", false=>"varval 2", :varname2=>"varval 3"}
irb(main):008:0> x.varname2 = 'varval 4'
=> "varval 4"
irb(main):009:0> x.to_h
=> {:varname1=>"varval 1", false=>"varval 4", :varname2=>"varval 3"}

one of many side effects is:

irb(main):009:0> y = x.dup
/var/lib/gems/3.0.0/gems/ostruct-0.5.0/lib/ostruct.rb:304:in `[]=': undefined method `to_sym' for false:FalseClass (NoMethodError)
Did you mean?  to_s

Start in lib/ostruct.rb at the []= call reported at line 304, and reproduce the Ruby 3.0 behavior with ostruct 0.5.0 using the examples in the issue. Compare repeated assignments and dup with stdlib OpenStruct; done means updated assignments retain the expected symbol key and dup no longer raises the shown NoMethodError.