Related docs:
https://docs.microsoft.com/en-us/office/vba/language/reference/user-interface-help/mid-statement

Reproduced, except for the result being ""abc 123 jhi"

IMHO this is not a bug, to make mid() vba compatible, use 

Option VBASupport 1

Sub Main
    msgbox my_function()
End Sub

function my_function () as string
   dim my_string as string
   let my_string = "abc def jhi"
   mid( my_string, 5 ) = "123"
   let my_function = my_string
end function

(In reply to Oliver Brinzing from comment #3)
> IMHO this is not a bug, to make mid() vba compatible, use 
> 
> Option VBASupport 1
> 
> Sub Main
>     msgbox my_function()
> End Sub
>...

On pc Debian x86-64 with master sources updated today, I got 
abc 123

with VBASupport equal to 1 or 0, whatever

With this patch:
diff --git a/basic/source/runtime/methods.cxx b/basic/source/runtime/methods.cxx
index fe266fdc009c..777b21040230 100644
--- a/basic/source/runtime/methods.cxx
+++ b/basic/source/runtime/methods.cxx
@@ -1159,7 +1159,7 @@ void SbRtl_Mid(StarBASIC *, SbxArray & rPar, bool bWrite)
 
                 OUStringBuffer aResultStr = aArgStr;
                 sal_Int32 nErase = nReplaceLen;
-                aResultStr.remove( nStartPos, nErase );
+                aResultStr.remove( nStartPos, nErase - nStartPos);
                 aResultStr.insert(
                     nStartPos, aReplaceStr.getStr(), std::min(nReplaceLen, nReplaceStrLen));

it works.
But this unit test fails:
    s = "The lightbrown fox"
    Mid(s, 5, 10, "lazy")
it expects:
"The lazy fox"
but with the patch I get:
"The lazyrown fox"

Perhaps, I could use "nErase - nStartPos" only when using VBASupport 1
but:
1) Still I don't know if the patch is right
2) Changing behaviour of mid function may bring some regressions for some other users

=> There's no more Basic expert in https://wiki.documentfoundation.org/FindTheExpert, so I'm a bit stuck.

I put in cc the bugtracker related to the QA test which fails with the test patch quoted in my previous comment.

(In reply to Julien Nabet from comment #4)
> On pc Debian x86-64 with master sources updated today, I got abc 123

sorry, only checked with LO 6.0.7.3 (VBASupport 1 worked but 0 crashes LO) and AOO 4.1.5, behaviour changed with LO 6.1.

maybe a side effect from fixes:

Bug 111313 - Crashed in Calc Macro (Basic)
Bug 121325 - 4-argument form of Mid() BASIC function broken in 6.x (worked in 5.x and OO)

Testing the initial case on Excel:
function my_function () as string
   dim my_string as string
   let my_string = "abc def jhi"
   mid( my_string, 5 ) = "123"
   let my_function = my_string
end function

Excel returns 
"abc 123 jhi"
not 
"abc 123 def"