x265 creates border artifacts

Using static libraries from ffmpeg 3.4 at Windows 10 x64, encoding a series of 8K x 8K images produces a video sequence which has strong artifacts at the borders . These artifacts can be reproduced using a synthetic 8K x 8K sequence.

Here is the libx265 coming from ffmpeg

x265 [info]: HEVC encoder version 2.5+27-0e168bdeb48b
x265 [info]: build info [Windows][GCC 7.2.0][64 bit] 8bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast LZCNT SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
x265 [info]: Main profile, Level-8.5 (Main tier)
x265 [info]: Thread pool 0 using 32 threads on numa nodes 0
x265 [info]: Slices                              : 1
x265 [info]: frame threads / pool features       : 6 / wpp(128 rows)
x265 [info]: Coding QT: max CU size, min CU size : 64 / 8
x265 [info]: Residual QT: max TU size, max depth : 32 / 2 inter / 2 intra
x265 [info]: ME / range / subpel / merge         : star / 57 / 3 / 3
x265 [info]: Keyframe min / max / scenecut / bias: 25 / 250 / 40 / 5.00
x265 [info]: Lookahead / bframes / badapt        : 30 / 8 / 2
x265 [info]: b-pyramid / weightp / weightb       : 1 / 1 / 1
x265 [info]: References / ref-limit  cu / depth  : 4 / on / off
x265 [info]: AQ: mode / str / qg-size / cu-tree  : 1 / 1.0 / 32 / 1
x265 [info]: Rate Control / qCompress            : CRF-28.0 / 0.60
x265 [info]: tools: rect amp limit-modes rd=6 psy-rd=2.00 rdoq=2 psy-rdoq=1.00
x265 [info]: tools: rskip limit-tu=4 signhide tmvp b-intra
x265 [info]: tools: strong-intra-smoothing lslices=4 deblock sao

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