I get the distinct feeling that "inbetween" occurs increasingly often as a single word, but I'm not at all clear on why it's used more in some contexts than others.
I had not previously been aware of seeing it printed other than as two words, but the practice seems not to be particularly new. The OED records the hyphenated noun in-between as meaning ‘(a) An interval. (b) A person who intervenes.’ The first citation is dated 1815:
He's fallen in love with Lady Naglefort, because she's an in-between.
It’s followed a year later in Jane Austen’s ‘Emma’ by:
Busy . . . talking and listening, and forming all these schemes in the in-betweens.
As an adjective meaning ‘placed between’, it occurs first, once again hyphenated, in 1898:
White or pale-coloured silk, with an in-between layer of chiffon.