`--hard-cbr` is not respected when frame size is set to 60 sometimes

[Yesterday17]

Version

opusenc opus-tools 0.2 (using libopus 1.3.1)
Copyright (C) 2008-2018 Xiph.Org Foundation

Command

ffmpeg -f lavfi -i anullsrc=channel_layout=5.1:sample_rate=48000 -t 1.201 silence.wav
opusenc --bitrate 64 --hard-cbr --music --framesize 60 --comp 0 --discard-comments --discard-pictures --save-range range silence.wav silence.ogg

Range file

2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
2880, 480, [[3, 157, 157, 157], MDCT, FB, S, 960, 16777216]
960, 160, [[1, 159], MDCT, FB, S, 960, 16777216]

Is it expected?