Btukfyl

I have an issue i can't figure it out, i have a definition like

letrendsplit@old=rendsplit@

defrendsplit@{%

  globalsetboxz@vbox{hbox{vbox{unvcopyz@}}}%

  rendsplit@old%

}

the equation:

begin{align}begin{split}{E}_{varepsilon }dot{x}left(vrule height 7.79312pt depth 0.15717pt width 0pttvrule height 7.79312pt depth 0.15717pt width 0ptright)&=Axleft(vrule height 7.79312pt depth 0.15717pt width 0pttvrule height 7.79312pt depth 0.15717pt width 0ptright)mathbin{+}Bvarphi left(vrule height 5.93326pt depth 3.39229pt width 0ptyvrule height 5.93326pt depth 3.39229pt width 0ptright)mathbin{+}{B}_{u}uleft(vrule height 7.79312pt depth 0.15717pt width 0pttvrule height 7.79312pt depth 0.15717pt width 0ptright),label{EEq1}end{split}tag*{$text{(1)}$}\[5pt]begin{split}yleft(vrule height 7.79312pt depth 0.15717pt width 0pttvrule height 7.79312pt depth 0.15717pt width 0ptright)&=Cxleft(vrule height 7.79312pt depth 0.15717pt width 0pttvrule height 7.79312pt depth 0.15717pt width 0ptright),label{EEq2}end{split}tag*{$text{(2)}$}end{align}

should appear aligned at equal sign but i noticed that no align at equal sign, but after fixing the definition above to be like

letrendsplit@old=rendsplit@

defrendsplit@{%

  globalsetboxz@vbox{unvcopyz@}%

  rendsplit@old%

}

the equations aligned correctly at the equal sign.
So my question is, why hbox command cause error in alignment.

Note: i use hbox to specify the x and y points before the inside vbox and after vbox by some special commands related to DVISVGM tool.

Thanks in advance

The original macro does

 vtop{%

            unvcopyz@

            globalsetbox8 lastbox

(it should not be doing global assignments to box 8 but ignore that for now)

this strips the last box (row) off the vertical list that was in box 0.

Your redefinition (which definitely should not be making global assignments to box 0) does

  globalsetboxz@vbox{hbox{vbox{unvcopyz@}}}%

which means that box 0 always just has a vertical list of exactly one box, with all the structure inside that box.

So the above code to remove the "last line" will always remove the whole thing.

Without any context, not possible to suggest how to fix this.