Weird rendering issues after grepping some text

In my .vimrc I have the line below, that I use for grepping for word under the cursor:

command! -nargs=+ Grp execute 'grep! -I -r -n --include=*.h --include=*.hpp --include=*.inl --include=*.cpp --include=*.c . -e <args>' | copen | execute 'silent /<args>'

Every time I run this command most part of the code disappears, and to see it I have to scroll down/up until the damaged regions are re-rendered.
Any ideas how to fix this?

Many thanks in advance!
PS. I'm not a vim guru, the command above was copy-pasted ;)

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