In the lyric of 余白 by 尾崎由香, there is the line
In this line, the 連用形 of the verb 引きこもる seems to be used as a 形容動詞. While it is quite common to use the 連用形 of a verb as a noun, this is the first time I see 連用形 used as a 形容動詞 if I remember correctly. In this particular case, I did some search and found many examples with "引きこもりのくせ", which is using the 連用形 as a noun in the usual way, but I am not sure whether there are any cases where the 連用形 can only be used as a 形容動詞.
So under what conditions can the 連用形 be used as 形容動詞 and can I tell it from the verb itself?