Also experienced the same issue. So I rolled back to beta 1 and all is working fine again. Then there was this update notice of beta 2. So I updated again to see what happens and the block indeed disappeared again. Instead I got to see the following message:
Your site does not support the “acf/testimonial” block. You can leave this block or delete it completely.
WP 4.9.8
GB 4.1.1
ACF 5.8.0. beta2
